Richardson, C.J., M.R. Walbridge, and A. Burns. 1988. Soil chemistry and phosphorus retention capacity of North Carolina coastal plain swamps receiving sewage effluent. NC Water Resources Research Institute. Raleigh, NC. Report Number 212.

Abstract: We analyzed the soil chemical properties and P sorption potentials of three North Carolina coastal plain swamps to (1) characterize their soil chemistries, (2) analyze both short- and long-term effects of wastewater addition on soil chemistry, (3) determine their P sorption capacities and the relationship between P sorption and soil chemistry and (4) develop a predictive index to evaluate the P sorption potentials of other coastal plain swamps. At Brown Marsh Swamp, soils were analyzed both before and after the initiation of wastewater discharge. At Cashie Swamp, which ahs been receiving wastewater additions for approximately 30 years, soils were collected both above and below the discharge point. Soils from nearby non-impacted Wahtom Swamp were analyzed for comparison.

Soils from all three swamps were characterized as acidic mineral soils with low total N:total P ratios typical of marsh and swamp ecosystems. Each soil, however, exhibited a unique combination of soil chemical parameters. NC coastal plain swamps exhibit variable soil chemistries despite their similar vegetational composition of tupelo gum, cottonwood, ash, and cypress.

Wastewater additions at Brown Marsh Swamp resulted in significant increases in soil pH, available PO4–P, extractable NH4–N, and total P, and significant decreases in microbial biomass PO4–P and extractable NO3–N. Changes were most significant in areas directly impacted by wastewater–the spray field and the main stream channel–and decreased both with increasing distance downstream and away from the main stream channel. The point source input of wastewater apparently impacted only a small portion of the total area of the swamp ecosystem.

Soil pH increased from 4.1 to 6.9 in the Brown Marsh spray field. Maximum bioavailability of phosphate in soils occurs at pH 6.5. Of an estimated P loading of 295 kg/ha of total P, 55 kg P/ha were stored in soil in the 0.2 ha Brown Marsh spray field during the first 1.5 years of wastewater additions. Approximately 47% (26 kg) of the stored P in these soils was present as relatively loosely bound extractable P. Extractable P concentrations in pre-treatment soils represented less than 1% of total soil P.

At Cashie Swamp, soils collected below the discharge point had significantly higher concentrations of extractable P, and significantly lower percent organic matter and concentrations of extractable Ca, Mg, and K, total N, total P, and N:P ratios than soils collected above the discharge point. These effects were primarily limited to an area within 150 m of the discharge point.

Laboratory estimates of P sorption potentials indicated differences in P sorption capacity among sites, and higher P sorption capacities in non-channel vs. channel soils. P sorption capacity was highly correlated (r2 = 0.96) with soil oxalate-extractable Al content. The resulting regression equation could be used to estimate the relative P sorption capacities of other NC coastal plain swamps being used or considered for use as receiving systems, based on a simple laboratory analysis of soil oxalate-extractable Al content.

We estimated that the Clarkton wastewater treatment plant discharges of 221 kg of PO4–P and 295 kg of total P annually into the 0.2 ha Brown Marsh spray field. Based on a modified Langmuir equation estimate of maximum P sorption potential (420 kg PO4–P/ha), each hectare of Brown Marsh Swamp soil could sorb nearly two years of annual phosphate discharge from the Clarkton plant in the upper 15 cm of the soil alone. Actual P storage in the Brown Marsh spray field, however, represents only about 4% of estimated annual discharge. High P loading rates (1000-1500 kg P/ha) and dramatic increases in soil pH are most likely responsible for the low efficiency of P removal observed in the Brown Marsh spray field. However, water quality data (Kuenzler 1987) indicate that the entire 4000 ha swamp was effective in P removal. Our calculations of maximum sorption potential suggest that this swamp could remove current loading rates for hundreds of years if wastewater P was discharged evenly over the entire swamp.

Maximum utilization of wetlands for phosphate removal from wastewater with minimum ecosystem impact can only be achieved under conditions which maximize retention time and the effective surface area of the wetland, and minimize the average impact per unit area. This could be achieved by adding acidified wastewater using a well-designed diffusion system, rather than as a point discharge. A diffusion system was not utilized at Cashie Swamp, while at Brown Marsh a poorly engineered diffusion (spray) system distributed wastewater over a relatively small (approximately 0.2 ha) portion of the wetland. Both sites exhibited severe overloading of nutrients in soils in the vicinity of the discharge point. Poor engineering design, not wetland retention capacity, was responsible for localized ecosystem failure.

Reproduced by permission

[Back to Publications]